Add campaign validation to ensure creatives are attached

[andrewmcodes]

Type of PR

• [ ] feature
• [ ] enhancement
• [x] bug fix
• [ ] other

Description

Make sure campaigns cannot be saved without creatives attached.

Checklist

• [x] My code follows the style guidelines of this project & the tests are passing
• [x] I have performed a self-review of my own code
• [x] I have requested a review from another developer(s)

[codecov[bot]]

Codecov Report

Merging #1314 into master will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1314   +/-   ##
=======================================
  Coverage   34.76%   34.76%           
=======================================
  Files         345      345           
  Lines        7989     7989           
=======================================
  Hits         2777     2777           
  Misses       5212     5212           

Impacted Files	Coverage Δ
app/models/campaign.rb	79.08% <100.00%> (ø)
app/jobs/schedule_organization_report_job.rb	86.20% <0.00%> (ø)